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#protein topology

3 public questions tagged with this topic.

Plasma Membrane Proteins

Which technique can be used to determine if a membrane protein is inside-out or right-side-out?

Determining orientation of membrane proteins after reconstitution into artificial vesicles essential for establishing vectorial transport competence. Proteoliposomes generated by removing detergent via dialysis may incorporate proteins randomly right-side-out exposing original extracellular domains outward or inside-out exposing cytoplasmic domains outward leading to mixed activity. Sidedness assay exploits membrane impermeability of antibodies one hundred fifty kilodaltons IgG unable to cross sealed bilayer. Intact vesicles incubated with monoclonal antibody against known extracellular epitope such as glycophorin N-terminus or Band 3 loop three undergo binding exclusively when epitope faces external medium detectable by secondary gold-conjugated antibody electron microscopy, flow cytometry, or after pelleting by immunoblotting pellet. Permeabilization with low concentration Triton X-100 zero point one percent exposes total epitopes providing total protein control. Alternative protease protection where extracellular trypsin cleaves only outward loops. Hydropathy plots predict number of spans not orientation in liposomes, SDS-PAGE separates subunits irrespective of sidedness, FRAP measures lateral mobility. Thus antibody labeling using impermeant probe provides definitive sidedness determination for inside-out versus right-side-out vesicles.

Ref: Rohde et al., Determination of Membrane Protein Orientation by Antibody Labeling, J Mol Biol Methods.

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Plasma Membrane Proteins

Which experimental technique is commonly used to determine the number of transmembrane domains in a protein?

Prediction of membrane topology begins with hydropathy profiling visualizing hydrophobic potential along polypeptide. Kyte-Doolittle scale assigns each amino acid numerical value reflecting water to vapor transfer free energy: isoleucine four point five, valine four point two, leucine three point eight highly hydrophobic, arginine minus four point five highly hydrophilic. Sliding window average typically nineteen to twenty one residues equivalent to hydrophobic thickness smooths profile revealing peaks above threshold about one point six indicating membrane-spanning segments. Positive-inside rule where arginine lysine enriched cytosolic loops aids orientation. Modern algorithms TMHMM Phobius TOPCONS integrate Hidden Markov Models with charge bias improving accuracy to about ninety percent. Experimental confirmation uses reporter fusions PhoA active periplasmic versus LacZ cytoplasmic, substituted cysteine accessibility method SCAM labeling membrane-impermeant reagents, and glycosylation mapping using acceptor sites inserted. SDS-PAGE separates by molecular weight, Western blotting detects immunoreactivity, FRAP measures lateral mobility but only hydropathy plot directly estimates number and positions of transmembrane hydrophobic segments guiding cloning and mutagenesis.

Ref: Kyte and Doolittle, A Simple Method for Displaying Hydropathic Character, J Mol Biol 1982.

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Protein Sorting and Trafficking

What determines the insertion orientation of a Type II membrane protein?

Insertion orientation of Type II single-pass membrane proteins, characterized by single non-cleaved signal-anchor yielding N-cytosolic C-lumenal topology, follows positive-inside rule first quantified in bacterial inner membrane proteins and validated in ER. Energetic penalty for translocating positively charged lysine and arginine across hydrophobic core is substantial, and cytosolic leaflet enriched in anionic phosphatidylserine attracts basic side chains, while Sec61 channel vestibule contains acidic residues influencing positioning. Therefore distribution of basic residues flanking signal-anchor strongly predicts orientation: more positively charged N-terminal flank favors N retention in cytosol corresponding to Type II, while more positive C-flank favors opposite Type III orientation with N-lumenal C-cytosolic. Experiments swapping charges by site-directed mutagenesis reverse topology, regardless of hydrophobicity length or downstream hydrophilic domain size. Hydrophilic domain properties and signal peptidase cleavage play no role because anchor is retained as transmembrane segment. Thus electrostatic bias around signal-anchor sequence serves as primary topological determinant for this class of membrane proteins guiding final arrangement in lipid bilayer and functional domain exposure. Additional coordination with cellular stress pathways ensures fidelity, prevents aggregation, and links trafficking to growth control and proteostasis maintenance across diverse cell types and developmental stages.

Ref: Hartmann et al., PNAS 86: 1989, Positive Charges Determine Type II Orientation.

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